Docs: fix Base64RawURL usage (#1336)

## Fixes Or Enhances
The usage of Base64RawURL was incorrect.

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- [x] Tests exist or have been written that cover this particular
change.

@go-playground/validator-maintainers

doc.go

@@ -959,7 +959,7 @@ Although an empty string is a valid base64 URL safe value, this will report
 an empty string as an error, if you wish to accept an empty string as valid
 you can use this with the omitempty tag.
 
-	Usage: base64url
+	Usage: base64rawurl
 
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